In order to be able to ensure interference-free functioning of electric circuits and the like, it is necessary to screen the latter against electromagnetic waves. For this purpose, the housing which surrounds the electric circuit must predominantly comprise metal housing parts. In this case, the screening prevents both penetration of electromagnetic waves and their escape from the housing. It is necessary for this purpose to establish between the metal housing parts continuously electrically conducting connections, in particular via contact spring elements. These connections must be as free from gaps as possible as a function of the level of the electromagnetic frequencies.
German application no. DE 36 04 860 A1 describes an arrangement for radio-frequency screening of the front plate of a withdrawable unit of a subrack. The front plate has a contact strip along one longitudinal side and, along the opposite longitudinal side, a retaining strip with an inserted sliding socket connector projecting beyond the edge of the front plate. In this arrangement, the front plate is designed in one piece with the contact strip and the retaining strip as an extruded section.
Because of the required inherent stability, the front plate must have an appropriate plate thickness. As a result, the minimum thickness and minimum height of contact strip and retaining strip are likewise prescribed. Because of the inwardly projecting contact and retaining strips, the printed circuit board fitted via a front connector on the rear of the front plate can be inserted only up to a minimum spacing in the direction of the front plate. Particularly in the case of printed circuit boards inserted in subracks, the insertion surface is, however, limited and cannot be extended at will. A problem in this case is the contact point between the sliding socket connector and a neighbouring contact strip, in which contact point is conditioned by the height of the sliding socket connector and is situated relatively deep in the housing. The regions of electric contact thereby displaced inwards cause a certain impairment of the electromagnetic screening of the housing. Electromagnetic waves can thus penetrate through the gaps between the front plates, and are absorbed by the contact strips and sliding socket connectors only relatively far behind the rear of the front plates. However, the further that electromagnetic waves penetrate into a housing, the greater is their possible interference potential.
It is an object of the present invention to specify a screening device for the electromagnetic screening of the housing which is more effective by contrast with conventional screening devices.
This object is achieved by means of a screening device for an electromagnetically screened metal housing, in particular for a subrack, having a base plate and at least one shaped strip which can be joined to a side edge of the base plate. The shaped strip has, furthermore, a strip-shaped contact limb, which is arranged approximately orthogonally relative to the plane of the base plate, and a strip-shaped bearing limb which can be joined to the base plate and is angled off in such a way as to make the shaped strip approximately flush with the front side of the base plate.
An advantage of the screening device according to the present invention is the low height of the separate shaped strips. The latter project only slightly into a printed circuit board fastened orthogonally, for example, on the rear of the screening device. This produces on the latter an optimum use of surface area for components.
It is particularly advantageous that in the case of two neighbouring screening devices the contact point between the spring shaped strip of the first screening device and the contact limb of the second screening device is situated virtually at the level of the rear of the base plates. This produces electrical contacts which extend virtually in one plane with the front side of the individual screening devices or other neighbouring housing parts and effects a virtually ideal electromagnetic screening of the housing.
A further advantage of the screening device in accordance with the present invention is the separately produced shaped strip, which can be designed as a spring shaped strip, for example. The separate production of a spring shaped strip permits, in particular, a finer working of the section, as such in the extrusion moulding process. It may then be possible to use a standard section and a standard spring shape for all contact socket connectors and shaped strips required in the electromagnetically screened housing. It is therefore advantageously possible, for example, to use only a single embodiment of a contact spring on a housing. In particular, it is possible to use a section shape onto which a contact socket connector can advantageously be plugged.